Glycoproteins produced by mammalian cell culture using recombinant DNA technology represent an important category of therapeutic pharmaceuticals for human health care. The biological functions of glycoproteins are often highly dependent upon their carbohydrate structures. Of the numerous sugars found in glycoproteins, the terminal sialic acid is considered particularly important. Sialic acid has been found to influence the solubility, thermal stability, resistance to protease attack, antigenicity, and specific activity of various glycoproteins. The amount of sialic acid in a glycoprotein is the result of two opposing process, i.e., the intracellular addition of sialic acid by sialytransferase activity and the extracellular removal of sialic acid by sialidase cleavage.
When terminal sialic acid is removed from serum glycoproteins, the desialylated proteins have significantly lower circulatory half-lives as compared to the sialylated counterparts. Removal of sialic acid from other glycoptoteins correlates with decreased biological activity and increased serum clearance. Additionally, during the production of glycoproteins by cells in batch culture, nutrient consumption and product accumulation may alter the cellular environment such that protein glycosylation decreases over time. Furthermore, the resultant glycoproteins tend to have glycoform heterogeneity and there is significant batch-to-batch variation in the production processes. Such changes are unacceptable in a bioprocess used for the large-scale production of protein therapeutic agents. Therefore, in mammalian cell culture systems it is often desirable to maximize the final sialic acid content of a glycoprotein product to ensure its quality and consistency as an effective pharmaceutical. A need exists, therefore, for a mammalian cell culture system that has enhanced glycoprotein sialylation. Such a system would improve the production of adequately sialylated recombinant glycoproteins.